It should be self-evident that any unsustainable practice cannot be sustained indefinitely. Since our civilization depends on many unsustainable practices, of which global warming is just one of the worst, it follows that our civilization cannot be sustained. Therefore, it will collapse. The only thing in doubt is how bad the collapse will be, and how fast.

Since the best estimates for a sustainable population on Earth are about 2 billion, whatever the collapse of civilization looks like, it will involve a lot of deaths.